Modern day multi-dynode photomultiplier tubes are often chosen for use in applications wherein some means must be employed to gate, i.e., to electronically turn the tube on or off. Manufacturers, recognizing this application requirement, have designed some of the latest state of the art photomultiplier tubes with an internal grid arrangement which tube data sheets usually state is to be used for the gating function. There are, however, additional design features of many of these tubes which significantly limit the effectiveness of utilizing the grid or grids for gating purposes. This invention and the method of its employment circumvents these limitations.
The photomultiplier tubes under consideration usually consist of a light-receiving photocathode followed by one or more grids, a series of typically up to 10 or more dynodes, and an anode. The last dynode may be laterally distributed in a form of up to as many as 100 dynodes. It is this last dynode or group of dynodes from which the tube output signals are typically taken.
Beyond the photocathode, at least one grid and the first several dynodes are usually slightly optically transparent. The first several dynodes all usually have some degree of photosensitive coating on them so that incoming photons can be converted to photoelectrons at any of these electrodes. This photosensitive coating limits the effectiveness of the grid structure in gating the tube such that the extinction ratio, namely the ratio of the output signal with the photomultiplier tube not gated to output signal with the photomultiplier tube gated, is limited in value.
For many applications, the extinction ratio is deliberately reduced to a level far below the value needed, solely for preventing the photomultiplier tube from saturating. Measures taken to limit the tendency of photomultiplier tubes to saturate, will consequently limit the tube's use in a particular circumstance. The ability to suppress saturation and re-gate the photomultiplier tube will considerably enhance the range of utility of any photomultiplier tube.